A resistive thermal printer typically comprises the following components: a thermal printhead having an array of selectively-activated thermal elements that can transfer dyes from a dye donor to a dye receiver in an imagewise fashion, a pressure interface or "nip" formed between a platen and the printhead which sandwich the dye donor and the dye receiver extending through the nip, a transport mechanism for transporting the dye receiver, a transport mechanism for transporting the dye donor, and electronics for mechanical and printhead control, as well as electronics for control of data path and image processing. The dye donor is normally supplied in rolls of yellow, magenta, cyan, and sometimes black color patches. The dye receiver may be in cut sheets or rolls of paper or transparency.
Various dye chemistries are used for thermal dye transfer printing. For example, it is known that a deprotonated cationic dye can be used for transfer to an acid-containing thermal receiver. In this case, the deprotonated cationic dyes are in neutral form. After the dyes are transferred to the thermal receiver, they undergo a protonation process during which the dye molecules are reprotonated by acidic moiety to become cationic. Such dyes that undergo deprotonation-reprotonation conversions are disclosed in detail in U.S. Pat. No. 5,523,274 titled "Thermal Dye Transfer System With Low-T.sub.g Polymeric Receiver Containing An Acid Moiety" issued Jun. 4, 1996 in the name of Leslie Shuttleworth, et al. This class of dyes is referred to herein as NONICAT dyes. In addition, the terminology dye conversion is used herein to refer to the protonation of the deprontonated cationic dye molecules of the NONICAT dyes. The dyes transferred to the thermal receiver are anchored onto and form a strong electrostatic bond with the acidic moiety in the receiver. The protonating action also causes a hue shift of the transferred dyes from a deprotonated dye form to a protonated dye form. In practice, it is desirable to complete the protonation process at a rapid rate. However, dye stratification readily occurs near the surface of a dye-receiving layer due to the previously mentioned dye-receiver protonating action which prevents subsequent dyes from easily diffusing into the dye-receiving layer. Moreover, a sizable amount of transferred NONICAT dyes stay near the surface of the dye-receiving layer and remain unprotonated. Hence, it is desirable to increase dye conversion rate and diffusion of dyes into the dye receiver when using NONICAT dye chemistry. Therefore, a problem in the art is slow dye conversion rate and slow diffusion of dyes into the dye receiver when using NONICAT dye chemistry.
Another problem associated with thermal resistive printing is inadequate protection against finger prints, dye retransfer, physical abrasion, and image instability. In the prior art, many dye receiving layers, unlike the above described layers using NONICAT dye chemistry, are hydrophobic. Such dyes can be readily dissolved by oil or grease present on the fingers of an operator of the printer. Thus, finger prints can easily form on a finished print. The finger print problem has historically been addressed by two methods. In the first method, a lamination layer is printed by the printhead on top of the transferred dye image as the last step in printing on the receiver. The lamination layer protects the dyes from being in direct contact with the surrounding environment. The disadvantage of the lamination method is the increased operation time for the printing process as well as additional cost of the media.
The second method of addressing finger prints formed on a finished print uses an additional heater to heat the receiver after printing. As disclosed in U.S. Pat. No. 4, 966,464 titled "Fusing Apparatus For Thermal Transfer Prints" issued Oct. 30, 1990 in the name of Robert J. Matoushek, such prior art heaters are in the form of heated fuser rollers that press against the thermal receiver. This pressurized heating causes the dye near the receiver surface to diffuse into the dye receiving layer, which diffusion reduces the probability of the dye coming into contact with external oil or grease. One disadvantage of these prior-art heaters and fuser rollers is that physical contact occurs between the heated roller and the dye receiver. In this regard, imaged dyes transferred to the dye-receiving layer may transfer back out and into the fuser roller. Thus, transfer of dyes into the fuser roller contaminates subsequent image printing. A further disadvantage associated with this prior art technique is that the heated pressure contact between the fuser roller and the printed image of the receiver produces image artifacts such as scratches and blisters (i.e., ruptured vapor bubbles) on the image-bearing surface of the receiver.
Therefore, there has been a long-felt need for providing a thermal resistive thermal printer apparatus and method that is free of the above cited disadvantages and problems.